It's been a while and I've not finished my Fetzila due to lack of time and it did take sometimes for me to collect all needed parts.

A few day ago, I started building the amp. Yesterday, after I finished soldering everything on the PCBs. I switched on the power, I use a variac and set the (main) input voltage at half (110V)(the transformer secondary voltage is only 12V AC) to see if something go wrong and actualy something did go wrong (I shoud have used a bulb in serie with the amp as someone advised on this topic). After I turned the power on, the toroid hummed loudly and it seemed that I got a short somewhere since I saw smoke come out below the PCB, from the position I came out, It seemed to come from the metal oxide resistors which are mounted underboard, it happended to both channels.
I turned off the power immediately to prevent further damage. I guess that I did something wrong. After that I tested the fuses on PCB and they all intact.

Attached here are the pics of my PCB. Coud you please take a look at them to see if I put everything in correct order.
As you could see, in the power supply section picture. I did not mount the diodes optimally as I soldered them with the metal tab facing in instead of facing out but I only realised it after soldering them in positions. Actually, I tried to separate them and not to let their metal tab to touch each other.

Thank you in advance.
ncc

Your rectifier diodes are soldered the wrong way the plastic case of the diodes must face the inside see pic.

Check your out transistor with meter and make sure the correct type is on proper position. If they are good then, try checking all the semis, after that I don't see why your amp behave as you mention. I will leave it to Hugh and the others to make suggestion.

When you have huge current flows like this, routinely check the following:

1. You have the correct output devices mounted, 1058 on left, 162 on right, not wrong way around.
2. Secondary #1 connected to LEFT side, positive rail, and Secondary #2 connected to RIGHT side, negative rail.
3. Diodes D1 and D2 correct way around.
4. Bias string connections all correct; D4, D5, D6 and P1. Check all these vital parts for continuity.

The problem is 95% likely to be in the output stage. The rectifier diodes have a common cathode at the center terminal, so can go in the board either way, tab forward, or tab rearward. Doesn't matter at all!

This is a ridiculously simple power amplifier and should not be too difficult to get going.

Good luck!

Thanks guys for helping NCC, he is being very patient, careful and calm!

After reading your advices, I dismounted the PCBs from the heatsinks and carefully checked everything and I could not find anything wrong:

- Output devices mounted correctly. I don't have 2sk1058/2sj162 so I use what I have on hand which is BUZZ900/905 pair, BUZZ900 on left, BUZ905 on right of the PCB
- Diodes D1 and D2 orientation is correct
- Bias string D4, D5, D6 connections all correct

I could not find anything abnormal, so mounted one channel on heatsink to test. I used a variac to increase voltage from about 50V to 220V and things seemed to be ok this time, I could set the bias and voltage across bootstrap resistor and output offset to about 5mv. Then I was able to do the same to the other channel (one channel at a time).

I puzzled me because I did not know what was wrong and I did not correct anything then everything seem to be fine.

After that I put everything back to the chassis then power on again and I got the same short like before. It took sometime for me to find out the problem:
I use only one transformer with two 25V secondary windings. Secondary #1 connected to LEFT side, positive rail of left channel but I inadvertently connect it to the RIGHT side negative rail of right channel, the same apply for the Secondary #2, that's should be the problem.

After correcting the problem, I could power two channels with out any problem. The amp is making beautiful music right now.

Thanks Hugh and everyone for helping

However as an inexperienced Diyer I do not understand why it has happended like that. If I power only one channel at a time it's fine but if I power two channels at the same time with my original wiring then I get a short.